1. Industrial Application Field
The present invention relates to a reflector of a vehicular headlight having a light-distribution control function, which is capable of forming a light-distribution pattern having a cutline specific to a low beam by effectively utilizing the entire reflecting surface without arranging a light-shielding member near a light source.
2. Prior Art
FIG. 57 shows the most basic construction of a vehicular headlight to produce a low beam light distribution which conforms to industry standards. As shown, a coil-like filament c is arranged near the focus b of a paraboloid-of-revolution reflector a so that the filament's central axis coincides with the optical axis of the reflector a (the optical axis is selected as the x-axis; a horizontal axis as the y-axis; and a vertical axis as the z-axis). This is called a C-8 type filament arrangement. Further, an outer lens d for light-distribution control is disposed in front of the reflector a.
Although the filament c is depicted in FIG. 57 as a cylinder with its front end being flat and its rear end (on the side of the focus b) having a pencil-like shape that is conical, this representation is just for convenience to clarify the direction of a projected image of the filament c. In the remainder of the disclosure, unless otherwise specified, the filament image should be considered as having a dimension only along the filament axis.
Reference character e designates a shade for forming a cutline. The shade e is disposed under the filament c, and serves to cut light rays directed to an approximate lower half a.sub.L of the reflector a as indicated by hatching in FIG. 58.
Thus, filament images formed by the reflector a become as shown in FIG. 59. And a pattern after being subjected to final light-distribution control by the outer lens d is as shown in FIG. 60.
FIG. 59 schematically shows the images of the filament c projected onto a screen disposed in front of the reflector a and away therefrom by a predetermined distance. In FIG. 59, "H--H" designates a horizontal line; "V--V", a vertical line; and "HV" an intersection of these lines.
As is understood from FIG. 59, since part of the light rays toward the reflecting surface are shielded by the shade e, the pattern without the use of the outer lens d assumes a fan-like shape (its central angle equals to 180.degree. plus the cutline angle) which is formed by removing the portion above the H--H line except for the cutline portion (indicated by the dashed line in FIG. 59). The light-distribution pattern of FIG. 60 is obtained as a result of light diffusion in the horizontal direction by the outer lens d.
By the way, the streamlining (i.e., reduction of the aerodynamic resistance coefficient) of car bodies has been demanded from the viewpoint of aerodynamics for automobiles. And as the so-called "slant-nose" design gains popularity, a headlight of the type in which the outer lens is considerably inclined with respect to the vertical axis, tends to be used to match this design.
As the angle formed by the outer lens with respect to the vertical axis, i.e., a so-called slant angle, is increased, the light-distribution control function of the outer lens can no longer be relied upon. More specifically, a long tailing phenomenon becomes conspicuous (in both right and left end portions of a light-distribution pattern) which is caused by wide-diffusing lens steps formed on the outer lens.
As a recent trend, this problem is solved by giving the light-distribution control function, which has been assumed by the outer lens, to the reflector.
Preference to a reflector having the light-distribution control function is also supported from the standpoint of accommodating a low bonnet structure. That is, in a car body design in which the height from a bumper to the front end of a bonnet is not large, it is preferable to provide a headlight whose vertical dimension is small. However, with this headlight, there exists a problem in the luminous flux utilization rate. That is, the technique of forming a cutline with a shade does not allow the luminous flux to be utilized effectively. Therefore, it is desired to form a cutline without using a shade. To respond to such a demand, there has been conceived an idea of forming a cutline by using the entire surface of the reflector and by relying only on the configuration of the reflector. This means giving the reflector the light-distribution control function.
Various types of reflectors having the aforesaid light-distribution control function have been proposed, each having unique features, such as configuration, focus position, etc. In one example, a reflecting surface is divided into a plurality of reflecting sectors, and the focuses of the respective reflecting sectors do not coincide with one another but are offset on the main optical axis of the reflector. This construction is disclosed in U.S. Pat. No. 4,772,988.
However such conventional reflectors, having a light-distribution control function, also have a certain limitation in a light-distribution pattern produced by the lower reflecting sectors. This tends to cause the quantity of light immediately below the horizontal line H--H to be relatively small, thereby imposing a problem in luminous intensity distribution.
To illustrate this point, let us assume the model in which a paraboloid-of-revolution reflecting surface as shown in FIG. 57 is divided into two sectors, i.e., upper and lower sectors. Also assume their focuses are offset forward and backward on the optical axis, causing the two sectors to have different focal lengths. Specifically, the focus of the upper half surface of the reflector is located near the rear end of the filament, while the focus of its lower half surface is located near the front end of the filament.
FIG. 61 shows a pattern f produced by the reflector a when the shade e is not used (the reflector a has a single focus b). The upper half surface and the lower half surface are not symmetrical. Since a portion contributing to the formation of a cutline is included in the upper half side, a pattern g by the upper half surface and a pattern h by the lower half surface is asymmetrical with respect to the H--H line.
FIG. 62 shows a pattern i obtained by a reflector having two focus positions. A pattern j produced by the upper half surface is identical, in shape, with the pattern g of FIG. 61, and is located in the same area. A pattern k produced by the lower half surface is identical, in shape, with the pattern h of FIG. 61 while 180.degree.-rotated around the intersection HV, thus being located under the horizontal line H--H.
As is understood from FIG. 62, since the quantity of light is relatively lower in a region A immediately below the horizontal cutline than in a region B where the patterns j and k are superposed, a brightness variation becomes gentler toward the cutline, making it difficult to form a sharp cutline.